1. Field of the Invention
The present invention relates to a rear projection screen. More particularly, the present invention is concerned with a rear projection screen having an improved surface structure and greatly improved optical properties.
2. Description of the Prior Art
Conventional rear projection screens can be roughly divided into the following four groups:
1. Micro-optical screens, in which an surface irregularity on the order of microns is provided on the surface of a transparent plate.
2. Dispersion screens, in which a light diffusing layer is formed by dispersing light scattering particles in a binder.
3. Wax screens, in which wax in the form of a sheet is used as a light diffusing layer.
4. Crystalline polymer screens, in which a crystalline polymer material in the form of a sheet is used as a light diffusing layer.
In the above micro-optical screen (1), a fine structure, e.g., Fresnel lens, fly's eye lens or a lenticular lens, is formed on the surface of a transparent plate, e.g., a glass plate or an acrylic resin plate, and a redistribution of light projected onto the screen is caused utilizing the refraction and scattering of light at the interface between the air and the transparent plate. A surface-matted screen with random and fine surface irregularity on its surface, e.g., ground glass, falls into this group.
In the dispersion screen (2), a powdered inorganic material such as SiO.sub.2, CaCO.sub.3, BaSO.sub.4, Al.sub.2 O.sub.3, ZnO, TiO.sub.2, glass, or the like, or a powdered organic material such as polymer or a latex is dispersed in an organic binder as is generally used in the paint industry and then coated on a transparent support, or is dispersed directly in the transparent support, light being scattered by these dispersed powders.
The wax screen (3) uses a diffusion plate in which a waxy solid such as natural wax, a petroleum wax or a synthetic wax is used as a light scattering material.
The crystalline polymer screen (4) uses a diffusion plate comprising a crystalline polymer, and desired optical properties are obtained by controlling by various methods the spherulite microstructure of the crystalline polymer.
Before discussing the advantages and disadvantages of conventional rear projection screens, characteristic values which are used to evaluate optical properties will be defined. Hitherto, various characteristic values have been proposed to evaluate the optical properties of rear projection screens. In the present invention, however, the optical properties of a rear projection screen will be evaluated by the use of the following four characteristic values.
A. Scintillation
Scintillation is caused by the aggregation of numerous spots of light appearing on a rear projection screen. The spots of light sparkle one by one and it appears as though the aggregation of light spots moves with the a movement of a viewer's eyes. Since scintillation fatigues the eyes of a viewer, it is most desired to reduce the same.
In a micro-optical element, as later described, light coming from each element of the micro-optical element is observed as an aggregation of light spots since they are separatedly and individually visible. While this phenomenon is due to a cause slightly different from that which causes scintillation, in the present invention, however, such light surface irregularity is included in scintillation.
There is no physical means to evaluate scintillation, as will be understood from the definition thereof, that is, scintillation cannot be expressed numerically, and thus scintillation is usually expressed using a trained individual's subjective measurement.
b. Image reproduction range
The image reproduction range is a characteristic value which shows the density range in which an image projected on a rear projection screen is reproducible, and is defined by the following equation: ##EQU1##
In this case, the maximum luminance varies not only with the characteristics of the screen itself, but also with the intensity of the projection light used to project an image on the screen, and, thus, strictly speaking, the intensity of the image projection light used should be specified.
On the other hand, the minimum luminance is equal to the luminance due to the reflection of room light on the side of the screen which faces an observer. With any rear projection screen, therefore, if it is used in the dark, the minimum luminance is substantially zero, and its image reproduction range becomes widened.
In general, however, since a rear projection screen is used in room light, a reduction to lower levels of minimum luminance through an increase in room light shielding properties is an important factor in providing a desired rear projection screen.
To increase the image reproduction range, it is required that the rear projection screen have a high diffusion transmittance and a low diffusion reflectance. When the former requirement is met, the maximum luminance on the screen increases, whereas when the latter requirement is met, reflection of room light on the surface of the screen decreases, thereby resulting in a decrease in minimum luminance, and, at the same time, loss at the rear of the screen of the image projection light from the projection light source decreases, thereby resulting in an increase in maximum luminance. Therefore, the image reproduction range as a whole increases.
c. Light redistribution characteristic
This term designates the degree of uniformity with which light is redistributed to the position of the observer (hereinafter referred to as the observation range) when image information is projected onto the screen. This term designates the degree of uniformity with which light is redistributed through a predefined audience space in such manner that its luminance, viewed from any point in the audience space, is substantially constant.
In general, it is desired that image information be distributed only in the observation range, and, furthermore, uniformly on the screen.
The following two characteristic values are used herein to show the above light redistribution characteristic.
The first characteristic value to show the diffusion characteristic of the screen is the scattering angle of scattering light (.theta.1/2), i.e. the angle at which the luminance decreases to 1/2 of that of light emitted vertically from the surface of the screen. The second characteristic value to show the uniformity of screen luminance is the ratio (R) of the luminance at the center of the screen to that at the edge of the screen, the luminance being measured by ISO, R782.
d. Resolving power
This term designates the number of lines per 1 mm which are resolvable on the screen. The resolving power of the screen should be determined upon considering the resolving power of the human eye, and thus the resolving power of the screen is desirably equal to or more than that of the human eye (7 to 10 lines/mm).
Hereinafter, the advantages and disadvantages of the four conventional types of rear projection screens will be explained using the above defined optical characteristic values.
With regard to scintillation, the micro-optical element screen is most inferior. This is considered to be caused by the facts that in this kind of screen each micro-element forms an independent image element, and the whole image comprises an aggregation of spots of light from these image elements, and that light from the projection light source is subjected to one or more scatterings or refractions at the interface between a transparent member having a micro-element structure and air, whose refractive indices are substantially quite different, thereby resulting in an uneven light distribution. Scintillation, therefore, is a characteristic of a micro-optical element screen, and, thus, it is impossible to reduce scintillation. On the other hand, the micro-optical element screen has the advantage that it's light redistribution characteristic can be controlled at will.
The dispersion screen is secondly high in scintillation. To decrease scintillation of this screen, it has been proposed that the difference in refractive index between the light scattering particles and the binder be reduced as much as possible, and, at the same time, the number of scattering particles per unit area, i.e., the particle density, be increased by decreasing the size of the particle. Such a method is described in Japanese Patent Application (OPI)2127/1971 and in U.S. Pat. No. 3,712,707. In accordance with this method, although scintillation can be decreased, other disadvantages are encountered, e.g., the diffusion characteristic increases excessively, thereby resulting in a decrease in resolving power, and, furthermore, the transmittance decreases due to an increase in the reflectance of the screen, thereby resulting in a considerable decrease in the image reproduction range.
In the dispersion screen, therefore, a decrease in scintillation and an increase in resolving power, diffusion characteristic, and image reproduction range cannot be simultaneously achieved, and thus it is impossible to produce a rear projection screen having excellent optical properties by increasing the optical characteristics thereof as a whole. Moreover, this dispersion screen is, due to its high diffusion characteristic, subject to the limitation that the thickness of the diffusion material should be controlled to about 100 .mu. or less when a resolving power of about 10 lines/mm or more is desired.
Furthermore, with both the micro-optical element screen and the dispersion screen, the image reproduction range is low, and it has been impossible to faithfully reproduce the wide image density region which a photographic film possesses.
On the other hand, the wax screen and the crystalline polymer screen are excellent in scintillation image reproduction range and resolving power.
In particular, the wax screen has excellent optical characteristics as compared with the other types of screens in that scintillation is very low and the image reproduction range is very broad.
These features of a wax screen are considered to be due to the fact that wax crystals exhibit complicated structures.
Wax can have complicated crystal shapes (e.g., twig shaped, needle shaped, plate shaped or block shaped, etc). Depending on the crystallization conditions, there is a small density difference (refractive index) between crystalline regions or between crystalline areas and non-crystalline areas, and the refractive index substantially continuously changes at the interface thereof, whereby incident light is passed through a complicated density zone in the light diffusion layer with multi-refraction and multi-scattering without total reflection, so that light is not reflected in the incident direction.
On the other hand, the crystalline polymer screen is inferior scintillation and image reproduction range to the wax screen. This is considered to be due to the fact that the length of a crystalline polymer increses with an increase in the molecular weight thereof, thereby resulting in an increase in melt viscosity, and that crystalline polymers do not have complicated crystalline structures at crystallization, and, in general, a micro-structure in which spherical crystals generally called spherulites are present is obtained. In order to remove these defects, an attempt to deform such a spherulite structure has been made, as is described in Japanese Patent Publication 19257/1973 and U.S. Pat. Nos. 3,573,141, 3,591,253 and 3,682,530. However crystalline polymer screens having optical characteristics as excellent as wax screens have not been obtained.
With respect to light redistribution, the micro-optical element screen is, as described above, superior. With the other types of screens, in general, only diffusing properties can be controlled, and it has been impossible to redistribute light onto a predetermined observation region.
The advantages and disadvantages of the four types of rear projection screens conventionally used will be understood from the above explanation. Among these screens, the micro-optical element screen and the dispersion screen are mainly used at present (see, for example, U.S. Pat. Nos. 2,180,113 and 2,480,031). With these screens, however, poor optical characteristics are achieved, and, thus, it has been impossible to produce a rear projection screen of these types having desired optical characteritics.
For this reason, attempts to combine the features of both such screens to make up for the defects thereof have been made. For example, Australian Patent 130,137 and Japanese Patent Application(OPI)2127/1971 describe coating or bonding a light diffusing material layer with light scattering particles dispersed in a binder onto a Fresnel lens plate and Japanese Utility Model Publication 7051/1973 describes embossing a lenticular structure on the surface of a light diffusing material layer having light scattering particles dispersed therein.
In the former method, however, scintillation is still not removed, and, in addition, a new problem occurs in that production is complicated due to joining two or more structures into one united body through coating or bonding. Further, resolving power is decreased. The latter method also fails to improve scintillation. In this method, the thickness of the diffusing material layer should be maintained at low levels in order to maintain the resolving power at a fixed level, and, thus, the thickness of the diffusing material layer varies relatively greatly as compared with the total thickness due to the micro-optical elements provided on the surface. As a result, the diffusing properties of the difussing material layer greatly change in a local fashion, and, finally, the micro-optical element changes the diffusing properties of the diffusing material layer.
On the other hand, although the wax screen possesses, as described above, excellent optical properties, it has been difficult to use on a practical basis due to its low mechanical strength. That is, since wax is very soft and brittle, it is difficult to produce a screen in the form of a sheet from wax alone. For this reason, it has been necessary to interpose wax between two transparent supports as described in Japanese Utility Model Publication 21110/1969. With a rear projection screen, however, since loss of light is caused at the interface between materials having different refractive indices, it is desired to minimize the number of interfaces. Thus, the above sandwich structure has the defect that the number of interfaces is large. In addition, such a structure cannot be produced with ease. Moreover, such a structure has the disadvantage that the wax peels off the transparent support or breaks with the passage of time due to the chemical inertness and brittleness of the wax itself. Furthermore, with such a structure it is difficult to control the light redistribution characteristic, and, thus, desired optical characteristics cannot be achieved. The inventor's research has revealed that in the case of providing a micro-optical element structure on the surface of a transparent support for the purpose of increasing the light redistribution characteristic, a considerable reduction in resolving power is encountered, and, thus, it has been impossible to use such a structure on a practical basis.
As a result of the inventors' research on wax screens, it has been found that it is possible to produce a wax screen of excellent mechanical strength and bonding properties by adding a wax modifying agent thereto as is described in Japanese Patent Application 446/1975, filed in the United States on Dec. 29, 1975 under Serial No. 644,683 in the names of Junji Miyahara and Hisatoyo Kato, hereby incorporated.
Even with such improvements, however, wax screens and crystalline polymer screens have an insufficient light redistribution characteristic, and many difficulties are encountered in producing such rear projection screens with desired optical characteristics.